The present invention relates to the production and processing of high Tc superconducting bismuth-strontium-calcium-copper-oxide materials.
Since the discovery of the copper oxide ceramic superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually. These materials have superconducting transition temperatures (Tc) greater than the boiling temperature (77K) of liquid nitrogen. However, in order to be useful for the majority of applications at 77K or higher, substantially single phase superconducting materials with high critical current densities (Jc) are needed. In general, this requires that the grains of the superconductor be crystallographically aligned, or textured, and well sintered together. Several members of the bismuth-strontium-calcium-copper-oxide family (BSCCO), in particular, Bi2Sr2CaCu2O8 (BSCCO 2212) and Bi2Sr2Ca2Cu3O10 (BSCCO 2223) have yielded promising results, particularly when the bismuth is partially substituted by dopants, such as lead ((Bi,Pb)SCCO).
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by a well-known process which includes the three stages of: forming a powder of superconductor precursor material (precursor powder formation stage); filling a noble metal container, such as a tube, billet or grooved sheet, with the precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more cores of superconductor precursor material in a surrounding noble metal matrix (composite precursor fabrication stage); and subjecting the composite to successive physical deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing). The alignment of precursor grains in the core (xe2x80x9ctexturedxe2x80x9d grains) caused by the deformation process facilitates the growth of well-aligned and sintered grains of the desired superconducting material during later thermal processing stages.
The general process, commonly known as xe2x80x9cpowder-in-tubexe2x80x9d or xe2x80x9cPITxe2x80x9d, is practiced in several variants depending on the starting powders, which may be, for example, metal alloys having same metal content as the desired superconducting core material in the xe2x80x9cmetallic precursorxe2x80x9d or xe2x80x9cMPITxe2x80x9d process, or mixtures of powders of the oxide components of the desired superconducting oxide core material or of a powder having the nominal composition of the desired superconducting oxide core material in the xe2x80x9coxide powderxe2x80x9d or xe2x80x9cOPITxe2x80x9d process. General information about the PIT method described above and processing of the oxide superconductors is provided by Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21 through 25, and the references cited therein.
OPIT precursor powders are prepared by reacting raw powders such as the corresponding oxides, oxalates, carbonates, or nitrates of the metallic elements of the desired superconducting oxide. Because the OPIT precursor powder is formed by chemical reaction, its actual phase composition will depend on the quality and chemical composition of the starting materials and on the processing conditions, such as temperature, time, and atmosphere. Different processing conditions will give rise to different phases or different ratios of phases. If secondary phases, such as calcium plumbate (Ca2PbO4), are formed in relatively large amounts, they can give rise to undesired effects. The presence of calcium plumbate, for example, disrupts the deformation induced texturing of the primary phase of the precursor powder, results in gas evolution during thermal processing, leads to growth of certain undesirable alkaline earth cuprate (AEC) phases which do not participate in the conversion of the precursor into the final oxide superconductor, and may induce undesired melting during heat treatments.
In order to avoid undesirable secondary phase formation, precursor powders sometimes are prepared by forming a BSCCO superconductor phase in a separate synthesis step and combining the BSCCO superconductor phase with a second metal oxide. The two powders may be readily reacted in a subsequent thermal processing step into the final oxide superconductor. By preparing the BSCCO superconductor in a separate reaction, it may be possible to avoid inclusion of undesirable secondary phases in the precursor powder.
A typical prior art preparation involves preparing essentially single phase (Bi,Pb)SCCO 2212 in a separate reaction and combining it with an alkaline earth cuprate. In subsequent thermal reactions, the two metal oxides react to form (Bi,Pb)SCCO 2223. The prior art reaction process is less than optimal because combining separate oxide powders necessarily reduces the intimate contact between the reactants (resulting in inhomogeneity), thereby requiring longer reaction times and/or harsher reaction conditions in order to obtain the final product. The slower reaction kinetics results in reduced control over the reaction process.
It is desirable, therefore, to have a method for preparing precursor powders having a controlled phase composition in a single step reaction process. It is further desirable to provide a method of controlling the phase composition of the precursor powder during its preparation and during subsequent thermomechanical processing.
The present invention provides a means of preparing a precursor powder for the BSCCO superconducting materials, particularly Pb-doped BSCCO materials, with selected primary and secondary phases and of controlling the phase composition of the precursor powder during its preparation and during subsequent thermomechanical processing steps. In general, in one aspect, the invention provides an improved precursor powder for the production of BSCCO superconducting material, and a process for making this precursor powder, while in another aspect it provides an improvement in processing of the precursor powder during the thermomechanical processing of the powder into the desired superconducting material.
In one aspect of the invention, a method for the production and processing of BSCCO superconducting material includes the steps of providing a mixture comprising raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO material, and heating said mixture at a selected temperature in an inert atmosphere with a selected oxygen partial pressure for a selected time period. The processing temperature and the oxygen partial pressure are cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase in the reacted mixture.
By xe2x80x9cfinal BSCCO superconducting materialxe2x80x9d, as that term is used herein, it is meant the chemical composition and solid state structure of the superconducting material after all processing of the precursor is completed. It is typically, though not always, the oxide superconductor phase having the highest Tc or Jc.
By xe2x80x9cdominant amountxe2x80x9d of the orthorhombic BSCCO phase, as that term is used herein, it is meant that the orthorhombic phase is the dominant phase present in the precursor powder, as selected among the members of the homologous BSCCO series of oxide superconductor. A xe2x80x9cdominant amountxe2x80x9d includes more than 50 vol %, preferably more than 80 vol %, and most preferably, more than 95 vol % of the members of the homologous BSCCO series as the orthorhombic phase.
Reference to the xe2x80x9corthorhombic phasexe2x80x9d recognizes the existence of two crystallographic structures for BSCCO superconducting materials, the tetragonal and the orthorhombic structures. The conversion of the tetragonal to the orthorhombic structure corresponds to the formation of an oxygen deficient structure. The conversion occurs simultaneously with the complete incorporation of a substituent having a variable oxidation state, i.e., Pb or Sb, into the BSCCO structure. Since the dopant typically exists in an oxide phase, such as for example, Ca2PbO4, the conversion from the tetragonal to the orthorhombic does not occur until all the dopant is consumed. Thus the formation of the orthorhombic phase is indicative of the complete reaction of the dopant carrier. The formation of the orthorhombic phase is indicated by the splitting of the XRD 200 and 020 peaks at 33xc2x0 (2xcex8).
In a preferred embodiment, the final superconducting material includes a BSCCO-2223 phase. In another preferred embodiment, the final superconducting material includes a (Bi,Pb)SCCO-2223 phase. In another preferred embodiment, the dominant orthorhombic phase includes a BSCCO-2212 phase. In yet another preferred embodiment, the dominant orthorhombic phase includes a doped BSCCO-2212 phase, where the dopant substitutes for bismuth. The dopant may be lead (Pb) or antimony (Sb), and is preferably Pb.
In a preferred embodiment, the processing temperature and the oxygen partial pressure are cooperatively selected to form an alkaline earth cuprate phase, in addition to the dominant orthorhombic BSCCO phase, during the heating step. By xe2x80x9calkaline earth cupratexe2x80x9d or AEC, as that term is used herein, it is meant metal oxide phases including an alkaline earth, such as calcium (Ca) and/or strontium (Sr) and including copper. There may be one or more phases present in the precursor powder. The overall composition of the AEC may vary as the oxidation states of the constituent elements vary and as calcium and strontium substitute for each other. Suitable AECs, include, by way of example only, (SrxCa1xe2x88x92y)CuOZ, (SrxCa1xe2x88x92y)2CuO3, (SrxCa14xe2x88x92x)Cu24O38. The AEC phases may also include single metal oxides, such as, by way of example, CuO, CaO and Cu2O.
In another preferred embodiment, the processing temperature and the oxygen partial pressure are cooperatively selected such that the oxygen partial pressure is below a value at which a Caxe2x80x94Pbxe2x80x94O phase is formed and above a value at which said dominant orthorhombic BSCCO phase decomposes. In yet another preferred embodiment, the heating step includes maintaining the temperature of the mixture in a range of 650xc2x0 C. to 795xc2x0 C. and the oxygen partial pressure in a range of 10xe2x88x925 atm O2 to 0.04 atm O2, and preferably maintaining the temperature of the mixture in a range of 720xc2x0 C. to 790xc2x0 C. and the oxygen partial pressure in a range of 10xe2x88x924 to 10xe2x88x922 atm O2.
In yet another aspect of the present invention, a (Bi,Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O superconducting material is prepared by providing a mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting (Bi,Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O material, and heating the mixture at a selected processing temperature in an inert atmosphere with a selected oxygen partial pressure for a selected time period, wherein the processing temperature and said oxygen partial pressure are cooperatively selected to eliminate, substantially, formation of a Caxe2x80x94Pbxe2x80x94O phase.
In a preferred embodiment, the processing temperature and oxygen partial pressure are cooperatively selected to form alkaline earth cuprate phases, in addition to eliminating, substantially, formation of a Caxe2x80x94Pbxe2x80x94O phase. In another preferred embodiment, the processing temperature and the oxygen partial pressure are cooperatively selected such that the oxygen partial pressure is below a value at which a Caxe2x80x94Pbxe2x80x94O phase is formed. In yet another preferred embodiment, the mixture is maintained at a temperature in a range of 650xc2x0 C. to 795xc2x0 C. and a oxygen partial pressure in a range of 10xe2x88x925 atm O2 to 0.04 atm O2 during the heating step. In yet another preferred embodiment, the mixture is maintained at a temperature in a range of 720xc2x0 C. to 790xc2x0 C. and a oxygen partial pressure in a range of 10xe2x88x924 atm O2 to 10xe2x88x922 atm O2 during the heating step.
In yet another aspect of the present invention, an elongated BSCCO or (Bi,Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O superconducting article is prepared. A mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO or (Bi,Pb)SCCO 2223 material is provided. The mixture is heated at a first selected processing temperature in an inert atmosphere with a first selected oxygen partial pressure for a first selected time period, such that the first processing temperature and the first oxygen partial pressure are cooperatively selected to form a dominant amount of an orthorhombic BSCCO or (Bi,Pb)SCCO 2212 phase in the reacted mixture. The reacted mixture is introduced into a metal sheath, and sealed. The sealed sheath is deformed to form an elongated precursor article of a desired texture. The orthorhombic BSCCO or (Bi,Pb)SCCO 2212 phase is heated after the deforming step at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period, such that second processing temperature and the second oxygen partial pressure are cooperatively selected to convert at least a portion of the orthorhombic BSCCO or (Bi,Pb)SCCO 2212 phase to the BSCCO or (Bi, Pb)SCCO 2223 superconducting material.
In a preferred embodiment, the first processing temperature and the first oxygen partial pressure are cooperatively selected to form a dominant amount of an alkaline earth cuprate phase, in addition to the dominant orthorhombic (Bi,Pb) SCC) 2212 phase. In another preferred embodiment, the deforming steps and second heating steps are repeated one or more times. In another preferred embodiment, the heating step includes cooperatively selecting the second processing temperature and the second oxygen partial pressure, such that oxygen partial pressure is below a value at which a Caxe2x80x94Pbxe2x80x94O phase is formed and above a value at which the dominant orthorhombic (Bi,Pb) SCC) 2212 phase decomposes. In yet another preferred embodiment, the heating step includes heating at a temperature in the range of 800xc2x0 C. to 845xc2x0 C. and preferably 800xc2x0 C. to 834xc2x0 C. and at an oxygen pressure in the range of 0.003 to 0.21 atm O2.
In yet another preferred embodiment, the second heating step includes ramping through a temperature range and an oxygen partial pressure range, such that the temperature and oxygen partial pressure range cooperatively include a value at which a Caxe2x80x94Pbxe2x80x94O phase is formed and/or a value at which the dominant orthorhombic (Bi,Pb)SCCO 2212 phase decomposes. The ramping is at a rate sufficiently rapid such that the formation of the Caxe2x80x94Pbxe2x80x94O phase and decomposition of the dominant orthorhombic (Bi,Pb)SCCO 2212 phase is kinetically disfavored. In a preferred embodiment, the ramp rate is greater than 0.1xc2x0 C./min and preferably is in the range of 0.1 to 100xc2x0 C./min.
The method of the present invention provides a precursor powder that substantially is free of undesirably secondary phases and which can be processed according to the above method of the invention to obtain a superconducting article having superior electrical properties.